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Full text of "Handbook Of Chemical Engineering - I"

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174                               CHEMICAL ENGINEERING
compressed air, a receiver is frequently equipped with a coil of pipes through which cooling water flows; in this way it serves as anaftercooler, and by precipitating the water from the air reduces difficulties in transmission lines and tools. The Ingersoll-Rand Co. states the amount of cooling water required for after-coolers for air at 80 to 100 Ib. pressure to be as follows:
Temperature of cooling water, degree F.............      50      60       70      80      90
Gallon per minute per 1,000 cu. ft. of free air per
minute......................................    120     140     160    189    200
When tlie transmission pipe line is long, receivers should be placed at both ends of the pipe.
Reheaters.—Heating the air just before expansion may increase the efficiency of the system, and in addition will increase the temperature at the end of expansion and prevent the freezing up of the motor. In quarry work stoves are sometimes used for preheating the air. In locomotive work for mines and surface use, hot water is frequently employed for this purpose. Reheaters are usually capable of raising the temperature of the air to from 300 to.500°F., although common practice shows temperatures of from 250 to 350°. In figuring on reheaters it is usual to assume that 1 Ib. of coal will give from 8,000 to 10,000 B.t.u. to the air. As the specific heat of air is approximately 0.24, 1 Ib. of coal will raise the temperature of approximately 100 Ib. (or 1,200 cu. ft.) of free air 300°.
The increase in efficiency resulting from reheating is greater with tools that use air expansively than with machines taking in air for full stroke. Sometimes it is not desirable to have the air entering a tool at a temperature above 300°, because of the effect of this high temperature on the lubrication. For these conditions small portable hot-water stove-type reheaters are available in capacities of from 62 to 800 cu. ft. of air per minute.
Lubrication.—If oil is fed too rapidly in the air cylinders there is a gradual accumulation of carbon, which interferes with the free movement of the valves and may actually choke the passages and produce high temperatures sufficient to produce ignition or explosion. Explosions have taken place from the introduction of kerosene or naphtha into the air cylinder for the purpose of cleaning the valves and cutting away the carbon deposits. This is a very effective way of cleaning valves and pipes, but is a source of danger and should be absolutely prohibited. Soft soap and water is the best cleanser for the air cylinder and is recommended even when the best grades of cylinder oil are used, feeding once or twice a week in order to prevent any gumming of the valves.
In order to reduce the danger of excessive temperatures, fusible safety alarm plugs may be inserted in the discharge line. These are usually set for a temperature of 350° for a single-stage compressor working at 40 Ib. gage pressure, for a two-stage compressor at 100 Ib. gage, and for a three- or four-stage compressor delivering at 1,000 Ib. gage. A 500° plug is furnished for use with a single-stage compressor discharging at 100 Ib. gage pressure.
Measurement of Discharge.—The relation between the piston displacement and the amount of air actually delivered, has been taken, in the past, from indicator cards, but this method is extremely inaccurate, since slippage due to leakage past the piston rings and leakage back into the cylinder through discharge valves, cannot be detected. As a matter of fact, these leakages tend to make an apparently better card and to increase the apparent volumetric efficiency. Recent